I would move the array to a class/module variable so that it's not instantiated every time.

EDIT:

What OP actually wants is to test if any of the strings "a3", "a4", "a5", "dl" is a sub-string of self.itemsize.downcase. The above tests if self.itemsize.downcase is equal to any of the strings, which is different from the former. Thanks clyfe for pointing that out.